Cover vs Stormsplitter?

[Brannigan]

Nah, cover should work with your mitigation. Like, for example, you can block covered hits. Damage does seem to bypass HG sometimes though - I think this mostly happens when you're getting hit by an AOE and taking two hits.
As for Stormsplitter, it's very possible that he dipped into its range - I'm decently sure it's a line AOE attack. If he did that while covering, he'd take 2 hits and get gibbed.